There are so many fossils of dinosaurs 70, 100, 200 million years ago, and no remains of human ancestors like Orrorin. If we assume that is size, but there are plenty of other examples of much smaller size than humans. What is the common explanation for this?
Three main reasons:
The timeline of interest for Homo Sapiens ancestors is very much shorter, on the order of 5 million years compared to the 180+ million years for dinosaurs.
The geographic range of that small set of species is very much smaller, restricted to a portion of central Africa compared to virtually the entire Earth for dinosaurs.
The number of species is very much smaller, being for proto-humans merely a subset of an already small taxonomic family, the Great Apes (Hominidae), whereas the dinosaurs are an entire clade of reptiles that spanned thousands, if not tens of thousands, of species.
The combination of these factors introduces perhaps 6 or more orders of magnitude difference in total population size over species lifetime. it is not surprising at all that there are vastly more fossils of dinosaurs than there are of hominids.
A better comparison than that of proto-humans to all dinosaurs would be the a comparison to a single small family of dinosaurs, of roughly the same size of humans and with a similarly restricted range. There are only few dinosaur species with a large number of fossils - but there were a huge number of total species over the 60 or so million years that they ranged the Earth.
Re: dinosaurs being reptiles - From my comments below:
My reading of the Physiology section of the The Wikipedia article on dinosaurs, which goes into extensive detail on that question, is that it comes down firmly that whether warm-blooded or cold-blooded, all known dinosaurs from more than ~60,000,000 years ago are in fact reptiles.
Note that from about 230 million years ago to the present, the "age of dinosaurs and birds" covers nearly half the entire time that multi-cellular animal life has existed on Earth (at ~600 million years). To think that any particular taxonomic group is unable to change a significant feature over that time is - naive in the extreme.